Researchers have studied various heat-expandable microcapsules (or heat-expandable microspheres) each comprising a shell of thermoplastic resin and a blowing agent which is encapsulated in the shell and gasifies at a temperature not higher than the softening point of the thermoplastic resin. For example, JP B 42-26524 discloses heat-expandable microcapsules and a basic production process thereof, and U.S. Pat. No. 3,615,972 discloses heat-expandable microcapsules having a shell of uniform thickness. Further, WO 99/43758 discloses heat-expandable microcapsules, which comprise a shell of thermoplastic resin in which functional groups are crosslinked, and a production process thereof.
Those heat-expandable microcapsules mentioned above have a problem that they fail to exhibit desirable expanding performance as the result of reduced maximum expansion ratio which is caused by heat history they undergo before their thermal expansion. They undergo heat history when they are stored at comparatively high temperature (though it is lower than a temperature at which they start thermal expansion) or are exposed to comparatively high temperature in mixing with resins which generate heat in the mixing operation. Thus those heat-expandable microcapsules should be stored at cool place, and they should be mixed with resins under a controlled mixing temperature so as to avoid the reduction of their thermal expanding performance such as expansion ratio which is caused by heat generated in the mixing.